The Irishman watched in satisfaction as the Black Cats secured their passage to a home fourth round tie against Middlesbrough with a calmly efficient performance, in which Derry winger James McClean capped another hugely promising display with a headed goal.

"I felt that in the first half we kind of allowed Peterborough to have the ball in their own half a bit too much, though we still created some great chances," said O'Neill.

"In the second we pressed them a bit closer to their goal and were more effective because of it."

Six weeks ago, this tie would have had all the makings of a potential upset. That, however, was before Sunderland sacked Steve Bruce and appointed O'Neill, whose immediate impact has been such that the Black Cats have won four and drawn one of the six Premier League games played under his aegis.

Unsurprisingly, December's Manager of the Month, O'Neill was clearly keen to maintain the newly-generated momentum, and made only two changes to the side which won at Wigan last Tuesday, Phil Bardsley returning from injury to replace Jack Colback at left-back, while Sebastian Larsson replaced Nicklas Bendtner in a 4-5-1 formation featuring Stephane Sessegnon as the solitary striker.

Helped by Peterborough's propensity to give the ball away, the visitors began to get on top from the start.

Larsson should have done better than pull his shot wide after Bardsley's pass inside the full-back put him clear down the right, but coming in at the far post he could hardly have failed to miss if Posh centre-half Ryan Bennett had not intercepted Craig Gardner's low cross.

Peterborough's strength is in going forward, however -- with 44 goals, Posh have the second best attack in the Championship this season -- and the match began to swing rapidly from end to end.

Paul Taylor, who began his senior career with Vauxhall Motors, showed why Barry Fry rates him so highly with a run from inside his own half and a shot which flew just wide.

The nearest the game came to a goal in the opening half hour came when McClean picked up Bardsley's driven cross and, from well out to the left of the penalty area, fizzed in a drive which clipped the top of the crossbar.

Crosses from the Sunderland right, where Bardsley and Larsson were combining effectively, were proving a serious problem for Peterborough.

Gardner's header from a Larsson centre almost caught out Posh goalkeeper Joe Lewis, and the first half ended with Lewis saving a Bardsley drive to his left.

Given Sunderland had enjoyed marginally the better of the first half, it could be argued they deserved the lead they were gifted at the start of the second.

Lining up a free-kick on the left, there was little doubt Larsson's low, curling delivery was intended as a cross, but the fact no-one got a touch deceived Lewis, who could only watch as it bounced inside his far post.

Their confidence now evident, Sunderland began to take control. Larsson, bursting into the penalty area on the right, saw his low drive beaten away by Lewis, but the second goal was not long delayed.

McClean's shot was deflected for a corner, and the 22-year-old rose to head Larsson's resulting delivery decisively past Lewis.More goals could have followed for the visitors as Peterborough, who had scored in each of their previous 18 games, began to lose heart.

"He has played three on the trot now and he's been brilliant," said O'Neill of McClean. "In my first week here he played against Manchester United in a reserve game that I thought was going to be called off because of 70mph winds.